the materials that you will need to construct this weapon are: a water bottle that is shaped like the picture. (needs cap as this will be your projectile.)
Step 2: Functions of the gun
To have this weapon fire correctly it will need air pressure built up in the top half of the bottle. to create this pressure, make sure that the cap is screwed on tightly; then take the bottom half of the bottle and twist it and try to get out all the air. Check your bottle with the picture below. ( it was very hard taking a picture while twisting the bottle at the same time, your gun will be much more functional and more powerful if you use 2 hands to twist the bottle.)
Step 3: FIRE!!!!
To fire this newly completed gun you should take one hand and hold the bottom half of the bottle to keep pressure in the top half of the bottle. With the other hand take your thumb and forefinger and untwist the the cap ever so slightly. Then when the cap is looser but still holding pressure take your thumb and very quickly untwist the cap and move your hand so that it does not interfere with the projectile's flight path. (Again, it was very hard to take the picture and untwist the cap at the same time so i tryed my best.)